Genetics MCB 215 PDF

COURSE CODE: BIO 201 MCB 215 COURSE TITLE: GENETICS COURSE UNIT: 2 GENETICS Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. It is the study of how genes bring about characteristics or tr...

COURSE CODE: BIO 201 MCB 215 COURSE TITLE: GENETICS COURSE UNIT: 2 GENETICS Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. It is the study of how genes bring about characteristics or traits in living things and how those characteristics are inherited. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of inheritance only began with the work of Gregor Mendel in the mid- nineteenth century. Although he did not know the physical basis for heredity, Mendel observed that organisms inherit traits in a discrete manner—these basic units of inheritance are now called genes. Genes are specific sequences of nucleotides that code for particular proteins. Through the processes of meiosis and sexual reproduction, genes are transmitted from one generation to the next. Augustinian monk Gregor Mendel developed the science of genetics. Mendel performed his experiments in the 1860s and 1870s, but the scientific community did not accept his work until early in the twentieth century because the principles established by Mendel form the basis for genetics, the science is often referred to as Mendelian genetics. It is also called classical genetics to distinguish it from another branch of biology known as molecular genetics. Mendel believed that factors are passed from parents to their offspring, but he did not know of the existence of DNA. Modern scientists accept that genes are composed of segments of DNA molecules that control discrete hereditary characteristics. Most complex organisms have cells that are diploid. Diploid cells have a double set of chromosomes, one from each parent. For example, human cells have a double set of chromosomes consisting of 23 pairs, or a total of 46 chromosomes. In a diploid cell, there are two genes for each characteristic. In preparation for sexual reproduction, the diploid number of chromosomes is reduced to a haploid number. That is, diploid cells are reduced to cells that have a single set of chromosomes. These haploid cells are gametes, or sex cells, and they are formed through meiosis. When gametes come together in sexual reproduction, the diploid condition is reestablished. The offspring of sexual reproduction obtain one gene of each type from each parent. The different forms of a gene are called Alleles. The set of all genes that specify an organism’s traits is known as the organism’s genome. The genome for a human cell consists of about 20,000 genes. The gene composition of a living organism is its Genotype. The expression of the genes is referred to as the Phenotype of a living thing. The two paired alleles in an organism’s genotype may be identical, or they may be different. An organism is said to be homozygous when two identical alleles are present for a particular characteristic. In contrast, the condition is said to be heterozygous when two different alleles are present for a particular characteristic. In a homozygous individual, the alleles express themselves. In a heterozygous individual, the alleles may interact with one another, and in many cases, only one allele is expressed. When one allele expresses itself and the other does not, the one expressing itself is the Dominant allele. The “overshadowed” allele is the Recessive allele. Dominant alleles always express themselves, while recessive alleles express themselves only when two recessive alleles exist together in an individual. TERMINOLOGIES 1. CHROMOSOME: Thread like structures present in nucleus are called chromosomes. At various stages of meiosis shape of chromosomes changes. For each kind of organism number, chromosome is the same 2. GENE: Is the unit of inheritance. It is a genetic factor (region of DNA) that helps determines a characteristics or traits. 3. ALLELES: are different forms of one type of gene i.e One of two or more alternate forms of a gene e.g T or t. 4. LOCUS: specific place on a chromosome occupied by an allele 5. PHENOTYPE: physical appearance or manifestation of a trait. Traits such as tall or dwarf are visible expression of the information contained in unit factors. 6. GENOTYPE: this is the genetic makeup of an organism which is either haploid or diploid. By reading the genotype we know the phenotype of individual. e. g. DD, Dd are tall and dd is dwarf. 7. Homozygous or pure lines: Presence of like alleles on corresponding loci of homologous chromosomes. It refers to organisms having identical alleles (TT or tt). 8. Heterozygous or hybrids: Presence of unlike alleles on corresponding loci of homologous chromosome. It refers to organisms with un-identical or different alleles (Tt). 9. Dominant: these refer to expressed trait. Out of two contrasting characters, the character expressed in first filial (F1) generation is known as dominant the phenomenon 10. Recessive: these refer to as unexpressed trait in an heterozygous. Out of two contrasting characters, the character that is not expressed in first filial (F1) generation is known as recessive HERITABLE AND NON-HERITABLE TRAITS The transmission or acquisition of characters or traits can occur from genetic transfer or environmental influence. The genetic transfer of trait from the parent to offspring is refers to heritable trait. Heritable trait or character by definition, are characteristics that are gained or predisposed to by an organism as a result of genetic transmission from its parents and will be passed to the organism's offspring while on the other hands character or trait acquired through environmental or external influence is refers to as Non-heritable. Non heritable traits by definition are characteristics that are gained by an organism after birth as a result of external influences or its own activities that change its structure or function and cannot be inherited. Visible or otherwise measurable properties of heritable traits are called phenotypes, while the genetic factors responsible for creating the phenotypes are called genotypes. The most basic question to be asked about a trait is whether or not the observed variation in the character is influenced by genes at all. It is important to note that this is not the same as asking whether or not genes play any role in the character development. Gene mediated developmental process lies as the basis of every character, but variation from individual to individual is not necessarily the result of genetic variation. Thus, the possibility of speaking a language at all depends critically on the structures of the central nervous systems as well as the vocal cords, tongues, mouth and ears which in turn depend on the nature of the human genome. There is no environment in which cows for example, will ever speak. Although, the particular language human speaks defers or varies from nation to nations, this variation is totally non genetic. Therefore, the question of whether or not a trait is heritable is a question about the role that differences in genes play in the phenotypic differences between individuals or groups. In principle, it is easy to determine whether any genetic variation influences the phenotypic variation among organisms for a particular trait. If genes are involved, then on the average the biological relatives should resemble each other more than the unrelated individuals do. This resemblance would be reflected as a positive correlation between parents and offspring between siblings. Parents who are larger than average would produce offspring that are larger than the average. The more seeds a plant produce the more seeds the siblings will produce also. Such correlations between relatives are however evidence of genetic variations only if the relatives do not share common environment more than the non-relatives do. It is absolutely fundamental to distinguish between familiarity and heritability at this point. Traits are familiar if members of the same family share them for what ever reasons. Inherited traits vary widely in complexity. Some appear in principles to be relatively limited. For example, human eye colour, which may either be brown or blue. Whiles some apparently are more complex. e.g the inheritance of the shape of the nose. Traits are heritable only if the similarity arises from shared genotypes. In general people who speak Yoruba have Yoruba parents and people who speak Igbo have Igbo parents, cross cultural activities over the years and the movement of people doing business at different locations in the country has demonstrated that this linguistic differences though familiar, are non genetic and non heritable. To determine whether a trait is heritable in human population, we must adopt studies that avoid the usual environmental similarities between biological relatives. Skin colour is clearly heritable as well as adult height but even in these traits also we have to be very careful. For example, the children of Japanese immigrants born in America are taller than there parents but shorter than the American average. So we might conclude that there are some influences of genetic differences. Yet there is also the effect of environmental cultural influences as second generation Japanese American are even taller than their American born parents. Personality traits, temperaments and cognitive performance (including IQ scores) and a whole variety of behaviors have been the subject of heritability studies in humans. Many showed familiarity. There is indeed a correlation between parents IQ and that of their children, but the correlation does not distinguish familiarity from heritability. To make that distinction requires that the environmental correlations between parents and children be broken. In summary example of heritable traits include, skin colour, eye colour, height, hair colours, shape of head, nose, etc while nonheritable trait include injury, hobby, language, behavior, skills etc. HISTORY OF GENETICS Although the science of genetics began with the applied and theoretical work of Gregor Mendel in the mid-1800s, other theories of inheritance preceded Mendel. A popular theory during Mendel's time was the concept of blending inheritance: the idea that individuals inherit a smooth blend of traits from their parents. Mendel's work disproved this, showing that traits are composed of combinations of distinct genes rather than a continuous blend. Another theory that had some support at that time was the inheritance of acquired characteristics: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children. Other theories included the pangenesis of Charles Darwin (which had both acquired and inherited aspects) and Francis Galton's reformulation of pangenesis as both particulate and inherited. The modern science of genetics traces its roots to Gregor Johann Mendel, a German- Czech Augustinian monk and scientist who studied the nature of inheritance in plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Naturforschender Verein (Society for Research in Nature) in Brünn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios. The importance of Mendel's work did not gain wide understanding until the 1890s, after his death, when other scientists working on similar problems re-discovered his research. William Bateson, a proponent of Mendel's work, coined the word genetics in 1905. After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1910, Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies. In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome. in 1928, Fredrick Griffith discovered the phenomenon of transformation. He discovered that dead bacteria could transfer genetic material to "transform" other still living bacteria. Sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and Maclyn McCarty identified the molecule responsible for transformation as the DNA. James D. Watson and Francis Crick determined the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin who showed that DNA had a helical structure (i.e., shaped like a corkscrew). Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder. This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. With this molecular understanding of inheritance, an explosion of research became possible. One important development was chain-termination DNA sequencing in 1977 by Frederick Sanger: this technology allows scientists to read the nucleotide sequence of a DNA molecule. In 1983, Kary Banks Mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of a DNA from a mixture. Through the pooled efforts of the Human Genome Project and the parallel private effort by Celera Genomics, these and other techniques culminated in the sequencing of the human genome in 2003. REPRODUCTION Reproduction is one of the key characteristics of living organisms. It is a biological process by which the parent organism gives rise to offspring similar to itself. Reproduction actually helps in the continuity of life and transmission of hereditary properties from the parent organisms to the offspring and from one generation to the other. Basically there are two types of reproduction, namely 1. Asexual reproduction and 2. Sexual reproduction ASEXUAL REPRODUCTION Asexual reproduction is a process by which an organism produces genetically similar or identical copies of themselves without the contribution of genetic materials from another organism. It is a type of reproduction in which a single parent give rise to an offspring with or without the involvement of gamete formation i.e it doesn’t involve fertilization thus only one parent organism is required. The offspring produced are not only identical to one another but are also of exact copies genetically to the parent organism i.e they possess the same nucleotide sequence as the parent cell. This mode of reproduction is common among unicellular organism including plant and animal with relatively simple organizational structure. There are various methods of asexual reproduction which include: 1. Budding: this is an asexual method of reproduction in which there is an outgrowth called bud from the parent organism which then separate from the parent organism or remain attached to the parent organism and grow into a new organism. This type of reproduction is found in yeast, coral and hydra. In coral for instance the new bud formed does not detach itself from the parent cell but multiply as part of a new colony but in hydra the bud separate itself from the parent cell and develop into an adult 2. Binary fission: this is the process by which a single cell divides to produce two equal daughter cells. During binary fission the parent cell undergoes cell elongation followed by DNA replication, the replicated DNA moves towards the edge of the cell after which the cell begins to inverge inward thus leading to the separation of the cell to produce daughter cells that are of equal size. This type of reproduction occurs in prokaryotic organisms such as bacteria 3. Fragmentation: this involves the breaking off of the body into parts called fragment which then develop into a new organism. The offspring formed by fragmentation vary in sizes. E.g sea star, annelids worm, poriferans etc. also some plant like liverworts contain structures like gemma which are specialized to reproduce by fragmentation 4. Spore formation: is a method of asexual reproduction which is found in non-flowering plants, fungi and some bacteria capable of producing spores. In this case the parent organism produces hundreds of spore which can grow into new offspring identical to the parent organism. Spores are microscopic, tough and resistant bodies which are usually round in shape and grow into a new organism under a suitable condition. The ability of an organism to produce spores allows them to thrive under adverse conditions and environmental fluctuation. Spores produced are disseminated by water, air, human and animal activity and the kind of spores produced is dependent on the type organism. 5. Vegetative reproduction: this is an asexual means of reproduction that occur in plant in which multicellular structures become detached from the parent plant and develop into new individuals that are genetically identical to the parent plant. Many plants naturally reproduce this way, but it can also be induced artificially. Horticulturalists have developed asexual propagation techniques that use vegetative plant parts to replicate plants. It involves the use of vegetative parts of the plants, such as leaves, stems, and roots to produce new plants or through growth from specialized vegetative plant parts, this is possible due to the presence of meristematic cells in the stem, root and leaves capable of cellular differentiation. Bulbs, corms, offsets, rhizomes, runners, suckers, and tubers are all important means of vegetative reproduction and propagation in cultivated plants  6. Parthenogenesis: this form of asexual reproduction where growth and development of embryo occur without fertilization i.e an egg develop into a new offspring without any form of fertilization. The resulting offspring can either be haploid or diploid. Example include bees, aphids, water flea, some ants, wasps, in some fishes etc. for instance bees uses parthenogenesis to produce diploid males (drones) and haploid females (workers) and if an egg is fertilized a queen is formed thus the queen determines the kind of bee to reproduce. SEXUAL REPRODUCTION Sexual reproduction on the other hand is a biological process that involves the formation of an offspring from the combination of genetic materials of two organisms. This type of reproduction involves the interplay between the male and female gamete. The genetic information is carried on chromosomes within the nucleus of specialized sex cells called gametes. In males, these gametes are called sperm and in females the gametes are called eggs or ova. During sexual reproduction the two gametes join together in a fusion by process known as fertilization, to create a zygote, which is the precursor to an embryo offspring. Each of the two parent organisms contributes half of the offspring’s genetic makeup by creating haploid gamete. The combination of these chromosomes produces an offspring that is similar to both its mother and father but is not identical to either. Phenotype traits, such as physical adaptations to an organism’s environment and genotype traits, such as resistance to disease, are passed down from each parent during sexual reproduction. Most organisms produced two types of gamete namely isogamete process known as Isogamy and anisogamete process known as Anisogamy or Heterogamy. Isogamy refers to sexual reproduction involving gametes of similar morphology found in most unicellular organisms. Therefore in isogamy: 1. Both gametes can be motile. This type occurs in algae such as some species of Chlamydomonas 2. Both gametes are non-flagellated  3. Conjugation: this is a more complex form of isogamy similar to the exchange of genetic material through a bridge in bacteria conjugation. This type of isogamy is seen in some green algae (Zygnematophyceae) e.g Spirogyra. These algae grow as filaments of cells so when two filaments of opposing mating types come close together the cell form conjugation tubes between the filaments. After which, one cell balls up and crawls through the tube into the other cell to fuse with it hence forming a zygote. This type is also found in the sexual life cycle of fungi. While anisogamy refers to sexual reproduction involving fusion of two dissimilar gametes i.e the gametes vary in size and/ or form, usually the smaller gamete is considered as the male while the larger as the female. The various forms of anisogamy include 1. Both gametes may be flagellated and hence motile 2. Both gametes may be non-flagellated. This occur in some algae and plants, in the red alga Polysiphonia non-motile eggs are fertilized by non-motile sperm. Similarly, in flowering plants, the gametes are non-motile cells within the gametophyte. The form of anisogamy that occurs in animals including humans is Oogamy (which is the fusion of large, immotile female gametes with small motile male gametes) where a large, non-motile egg (ovum) is fertilized by a small motile sperm (spermatozoon). There are various types of sexual reproduction namely: Allogamy This is the fusion of gametes during fertilization arising from two different individuals i.e the female gamete called the egg or ovum fuses with the male gamete called the sperm this is otherwise known as cross fertilization. Both egg and sperm are cells specialized to perform the task of reproduction; each cell contains only 23 chromosomes (these are called haploid cells) rather than the normal 46 chromosomes present in other cells of the body. The two haploid cells fuse together to create a diploid cell which then undergoes mitosis, in order to grow and form an individual organism. Mitosis is the division of one cell into two, after the DNA has been replicated within the nucleus. Autogamy Autogamy also known as self-fertilization or self-pollination is the fusion of male and female gametes which are produced by a single individual. Species which are able to produce both male and female gametes are called hermaphrodites. Although autogamy is similar to asexual reproduction, in that there is no input of genetic diversity from a partner, the recombination of chromosomes from the male and female gametes results in offspring with slightly altered genetic information, which can therefore look phenotypically different from their parents. Most plants and earthworms reproduce by autogamy. It is sometimes possible for hermaphrodites to reproduce with other hermaphrodites. In this case, genetic diversity does increase within the population. Internal Fertilization Internal fertilization is the fertilization of the egg by the sperm within the body of one of the parents, usually by means of sexual intercourse. Internal fertilization usually takes place within the female body, after the male implants sperm. However there are exceptionally rare examples, such as seahorses (Sygnathidae), where the female implants her eggs into the male and the zygote is formed within the male’s body External Fertilization External fertilization is the fertilization of the egg by the sperm outside the body of the parent. Most amphibians and fish and many invertebrates use external fertilization, producing anything from hundreds to billions of gametes at a time into close proximity. The quick release of gametes into aquatic environments called spawning. However, sometimes females will lay eggs on a particular substrate which are subsequently fertilized by males. The sex cells of creatures which reproduce through external fertilization often have special adaptations for movement, such as the addition of strong flagella for independent movement.  CHROMOSOME AND ITS STRUCTURE A chromosome is an organized structure of DNA and protein that is found in cells. A chromosome is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek chroma - color and soma - body due to their property of being very strongly stained by particular dyes. Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of 10,000 to 1,000,000,000 nucleotides in a long chain. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Today we know that a chromosome contains a single molecule of DNA along with several kinds of proteins. A molecule of DNA, in turn, consists of thousands and thousands of subunits, known as nucleotides, joined to each other in very long chains. A single molecule of DNA within a chromosome may be as long as 8.5 centimeters (3.3 inches). To fit within a chromosome, the DNA molecule has to be twisted and folded into a very complex shape. Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes. Viral Chromosomes The chromosomes of viruses are called viral chromosomes. They occur singly in a viral species and chemically may contain either DNA or RNA. The DNA containing viral chromosomes may be either of linear shape (e.g., T2, T3, T4, T5, bacteriophages) or circular shape (e.g., most animal viruses and certain bacteriophages). The RNA containing viral chromosomes are composed of a linear, single-stranded RNA molecule and occur in some animal viruses (e.g., poliomyelitis virus, influenza virus, etc.); most plant viruses, (e.g., tobacco mosaic virus, TMV) and some bacteriophages. Both types of viral chromosomes are either tightly packed within the capsids of mature virus particles (virons) or occur freely inside the host cell. Prokaryotic Chromosomes The prokaryotes usually consist of a single giant and circular chromosome in each of their nucleoids. Each prokaryotic chromosome consists of a single circular, double-stranded DNA molecule; but has no protein and RNA around the DNA molecule like eukaryotes. Different prokaryotic species have different sizes of chromosome. Eukaryotic Chromosomes The eukaryotic chromosomes differ from the prokaryotic chromosomes in morphology, chemical composition and molecular structure. The eukaryotes (plants and animals) usually contain much more genetic information than the viruses and prokaryotes, therefore, contain a great amount of genetic material, DNA molecule which here may not occur as a single unit, but, as many units called chromosomes. Different species of eukaryotes have different but always constant and characteristic number of chromosomes. In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The shape of the eukaryotic chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. They are thin, coiled, elastic, contractile thread-like structures during the interphase (when no division of cell occurs) and are called chromatin threads which under low magnification look like a compact stainable mass, often called as chromatin substance or material. During metaphase stage of mitosis and prophase of meiosis, these chromatin threads become highly coiled and folded to form compact and individually distinct ribbon-shaped chromosomes. These chromosomes contain a clear zone called kinetochore or centromere along their length. Eukaryotes (cells with nuclei such as plants, yeast, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes. The number and position of centromere is variable, but is definite in a specific chromosome of all the cells and in all the individuals of the same species. Thus, according to the number of the centromere the eukaryotic chromosomes may be acentric (without any centromere), mono centric (with one centromere), dicentric (with two centromeres) or polycentric (with more than two centromeres). The centromere has small granules or spherules and divides the chromosomes into two or more equal or unequal chromosomal arms. According to the position of the centromere, the eukaryotic chromosomes are divided into Metacentric: centromere is in the middle meaning the p and q arms are of comparable length e.g chromosomes 1, 3, 16, 19, 20 Submetacentric: centromere is off centre leading to shorter p arm relative to the q arm (e.g chromosomes 2, 4-12, 17, 18, X) Acrocentric: centromere severely off-set from the centre leading to much shorter p arm (e.g chromosomes 13-15, 21, 22, Y) Telocentric: centromere found at the end of the chromosome, meaning no p arm exists (chromosome not found in humans) A: Metacentric, B: Sub metacentric, C: Acrocentric, D: Telocentric During the cell divisions the microtubules of the spindle are attached with the chromosomal centromeres and move them towards the opposite poles of cell. Beside centromere, the chromosomes may bear terminal unipolar segments called telomeres. Certain chromosomes contain an additional specialized segment, the nucleolus organizer, which is associated with the nucleolus. CHEMICAL STRUCTURE OF CHROMOSOMES Chemically, the eukaryotic chromosomes are composed of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), histone and non-histone proteins and certain metallic ions. The histone proteins have basic properties and have significant role in controlling or regulating the functions of chromosomal DNA. The non-histone proteins are mostly acidic and have been considered more important than histone as regulatory molecules. Some non-histone proteins also have enzymatic activities. The most important enzymatic proteins of chromosomes are phosphoproteins, DNA polymerase, RNA-polymerase, DPN-pyropbosphorylase, and nucleoside triphosphatase. The metal ions as Ca+ and Mg+ are supposed to maintain the organization of chromosomes intact TERMS Genome: the complete set of genetic information an organism has in its DNA Chromosome: threadlike structure of DNA and protein containing genetic information Homologous chromosome: set of chromosomes (one from each parent) that are very similar to one and another and have the same size and shape Diploid (2n): cell that contains two sets of homologous chromosomes Haploid (n): cells that contain only a single set of genes Sex chromosome: one of two chromosomes (x or y) that determines an organism’s sex Autosome: chromosome that is not a sex chromosome Karyotype: micrograph image of diploid set of chromosomes grouped in pairs CHROMOSOMES NUMBER During cell division in organism, the number of chromosomes can either occur as diploid or haploid, most eukaryote are diploids, i.e., each somatic cell of them contains one set of chromosomes inherited from the maternal (female) parent and a comparable set of chromosomes (called homologous chromosomes) from the paternal (male) parent. The number of chromosomes in a dual set of a diploid somatic cell is called the diploid number (2n). The sex cells (sperms and ova) of a diploid eukaryote cell contain half the number of chromosomal sets found in the somatic cells and are known as haploid (n) cells. A haploid set of chromosome is also called genome. The fertilization process restores the diploid number of a diploid species. Different species have different numbers of chromosomes human for instance has 46chromosomes, dogs have 78. For example, humans are diploid and have 46 chromosomes in their somatic cells. These 46 chromosomes are organized into 23 pairs: 22pairs are autosome and the 1 pair is the sex chromosomes. The sex cells of a human are haploid containing only one homologous chromosome from each pair. This is so that when the sex cells fuse together during fertilization, a complete diploid set is formed. CELL CYCLE AND CELL DIVISION Cell division is a very important process in all living organisms. It is a process in which cells reproduce their own kind. The growth, differentiation, reproduction and repair take place through cell division. There are two types of cell division namely Mitosis (Vegetative cell division) and Meiosis (reproductive cell division). During the division of a cell, DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA replication and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. The cell cycle is divided into two basic phases namely: 1. Interphase 2. M-phase (Mitosis phase) Interphase: The interphase though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. The interphase is divided into three further phases: 1. G1 (Gap 1)  2. G2 (Gap 2) 3. S-phase (synthesis phase) G1 phase corresponds to the interval between mitosis and initiation of DNA replication. During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA. S or Synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G2 phase, proteins are synthesized in preparation for mitosis while cell growth continues. M-PHASE: It is a short phase. It includes two important processes that occur simultaneously. They are Karyokinesis (division of the nucleus) and Cytokinesis (division of the cytoplasm), resulting in two daughter cells. After ‘M’ phase the cell may enter either Interphase to repeat the cell cycle or G0 phase (Quiescent stage) to arrest cell cycle. Then the cells in G0 phase may grow and differentiate into different cell types to perform different functions MITOSIS Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour. The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells. In some single-celled organism, mitosis forms the basis of asexual reproduction. In diploid multicellular organism sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumor formation. Mitosis, although a continuous process, is conventionally divided into four stages: prophase, metaphase, anaphase and telophase a. Prophase Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organize the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere. b. Metaphase: The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis; hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This is the stage at which morphology of chromosomes is most easily studied. At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell. Hence, the metaphase is characterized by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.  c. Anaphase: At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. d. Telophase: At the beginning of the final stage of mitosis, i.e. telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles. This is the stage which shows the following key events: a. Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. b. Nuclear envelope assembles around the chromosome clusters. c. Nucleolus, golgi complex and ER reform. Function of Mitosis 1. Development and growth: The number of cells within an organism increases by mitosis. This is the basis of the development of a multicellular body from a single cell, i.e., zygote and also the basis of the growth of a multicellular body. 2. Cell replacement: In some parts of the body, e.g. skin and digestive tract, cells are constantly sloughed off and replaced by new ones. New cells are formed by mitosis and so are exact copies of the cells being replaced. In like manner, red blood cells have a short lifespan (only about 4 months) and new RBCs are formed by mitosis. 3. Regeneration: Some organisms can regenerate body parts. The production of new cells in such instances is achieved by mitosis. For example, starfish regenerate lost arms through mitosis.  4. Asexual reproduction: Some organisms produce genetically similar offspring through asexual reproduction. For example, the hydra reproduces asexually by budding. The cells at the surface of hydra undergo mitosis and form a mass called a bud. Mitosis continues in the cells of the bud and this grows into a new individual. The same division happens during asexual reproduction or vegetative propagation in plants. Mitotic Errors Errors can occur during mitosis, especially during early embryonic development in humans. Mitotic errors are mistakes that happen during mitosis which leads to the production of daughter cells with too many or too few chromosomes, a feature known as aneuploidy (a condition associated with caner). Early human embryos, cancer cells, infected or intoxicated cells can also suffer from pathological division into three or more daughter cells (tripolar or multipolar mitosis), resulting in severe errors in their chromosomal complements. MEIOSIS Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome). The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids. First Meiotic Prophase: Leptotene: Chromosomes condense, thicken and begin to coil up and become visible. Alternating thick areas called chromomeres begin to appear on each chromosome. Zygotene: At this stage, homologous chromosomes pair up (Synapsis) to form bivalents. Homologous sex chromosomes do not form bivalent but are only joined at the tips of their short arms Pachytene: Chromosomes are much thicker and more pronounced. The chromatids of pairing chromosomes are also visible as a tetrad of four strands. Diplotene: Paired chromosomes begin to disengage and move apart. Areas at which exchange of materials would have occurred between homologous pairs (Chiasmata) are also becoming visible. At the end of this stage the Chiasmata terminalize i.e. Draw to the end of the chromosome arm. Diakinesis: Final terminalization of Chiasmata occur bringing prophase of first meiotic division to an end. First Meiotic Metaphase: The nuclear membrane disintegrates and chromosomes migrate to the equatorial plane of the dividing cell. As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This attachment is referred to as a bipolar attachment. First Meiotic Anaphase: Bivalent chromosomes disjoin and each member migrates randomly to opposite poles of the cell. Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. In meiosis only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin which prevents the sister chromatids from separating. This allows the sister chromatids to remain together while homologs are segregated. This is the stage at which random and independent assortment of paternal and maternal chromosomes occurs. First Meiotic Telophase: Chromosomes are assembled and enclosed in nuclear membrane on either side of the centre of the cell. The cytoplasm is divided into two (Cytokinesis) leading to the formation of two daughter cells from one mother cell. Each daughter cell now has the haploid number of chromosome (23 in the human)  STAGES OF MEIOSIS I MEIOSIS II Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact. Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids. Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell. Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells.  STAGE OF MEIOSIS II Errors in Meiosis Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes i.e. having a missing or extra chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities. Examples of developmental disabilities include autism spectrum disorder (ASD), cerebral palsy (CP), attention deficit hyperactivity disorder (ADHD) and learning disabilities. DIFFERENCES BETWEEN MEIOSIS AND MITOSIS Meiosis Mitosis End result Normally four cells, each with half the Two cells, having the number of chromosomes as the parent same number of chromosomes as the parent Function Production of gametes (sex cells) in Cellular reproduction, sexually reproducing eukaryotes with growth, repair, asexual diplont life cycle reproduction Where does it happen? Almost all eukaryotes (animals, plants, All proliferating cells in all fungi, and protists); In gonads, before eukaryotes gametes (in diplontic life cycles); After zygotes (in haplontic); Before spores (in haplodiplontic) Steps Prophase I, Metaphase I, Anaphase I, Prophase, Prometaphase, Telophase I, Prophase II, Metaphase II, Metaphase, Anaphase, Anaphase II, Telophase II Telophase Genetically same as No Yes parent? Crossing over happens? Yes, normally occurs between each pair Very rarely of homologous chromosomes Pairing of homologous Yes No chromosomes? Cytokinesis Occurs in Telophase I and Telophase II Occurs in Telophase Centromeres split Does not occur in Anaphase I, but occurs Occurs in Anaphase in Anaphase II ALTERNATION OF GENERATIONS Alternation of generations otherwise known as metagenesis is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether or not these forms are free-living. It is the type of life cycle that occurs in plants and algae in the Archaeplastida and the Heterokontophyta that have distinct haploid sexual and diploid asexual stages. In these groups, a multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n. The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a zygote, which develops into a diploid sporophyte. This cycle, from gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many algae undergo sexual reproduction. The relationship between the sporophyte and gametophyte varies among different groups of plants. In those algae which have alternation of generations, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. Alternation of generations has several distinct features, and these features can be slightly modified between species. In general, the generations alternate between the sporophytes capable of creating spores and the gametophytes, capable of creating gametes. ALTERNATION OF GENERATION LIFECYCLE  Sporophyte To form a sporophyte, two haploid gametes come together to form a diploid zygote. Typically, haploid organisms are defined by having an “n” number of chromosomes. When two gametes of the same species come together, each has n chromosomes. Therefore, the diploid zygote which forms is considered to have 2n worth of genetic material, or exactly twice as much. Not only is there twice as much DNA, but it represents codes for the same proteins in the same organism. The sporophyte is a multicellular organism formed from multiple rounds of mitosis on the zygote. Thus, the sporophyte individual remains a 2n organism. Then, when the sporophyte reaches maturity, a key point in the alternation of generations takes place. The sporophyte develops organs, known as sporangia. These specialized reproductive organs are used to create single-celled, haploid spores. These cells will be released into the air or water and carried away. When they reach a suitable environment, they will begin the process of developing into the gametophyte.  Gametophyte This represents the next generation in the alternation of generations, as the haploid spore is created. The spore is technically a new organism, and has only half the DNA as the parent organism. This spore will undergo successive rounds of meiosis to form a new multicellular individual, the gametophyte. Where the sporophyte generation creates spores, the gametophyte generation creates gametes. Gametes are produced by special organs on the gametophyte, the gametangia. These gametes are then broadcast into the environment, or transferred between plants. Upon finding an opposite gamete, they begin the process of fusing to form another zygote. This zygote will eventually become a sporophyte, and the alternation of generations will keep turning. While this is a simplistic version of the alternation of generations, there are many complexities. Because of these complexities, and because all plants undergo some version of alternation of generations, scientists prefer to refer to other aspects of their reproductive cycles to define the species. The simplest form of alternations of generations is found in the fern. The large, leafy fern is the diploid organism. On the undersurface of its fronds or leaves, its cells undergo meiosis to create haploid cells. However, these cells do not immediately unite with others to recreate the diploid state. Instead, they are shed as spores and germinate into small haploid organisms. Because the diploid organism creates spores, it is called the sporophyte generation of the life cycle. Upon reaching maturity, the haploid organism creates haploid egg and sperm cells (gametes) by mitosis. Because the haploid organism creates gametes, it is called the gametophyte generation of the life cycle. The male gametes (sperm) are then released and swim to the female egg. Fusion of the gametes creates the new diploid sporophyte, completing the life cycle. Whereas the fern gametophyte and sporophyte generations are completely independent, in some types of plants one generation lives on or in the other and depends on it for nutrition Alternation of generation in ferns TRANSMISSION OF HEREDITARY CHARACTER Genetic information can be transferred from organism to organism basically by two main methods: 1. Vertical gene transfer: this is the transfer of genetic materials from mother cell to daughter cell during cell division. This type of genetic transfer is mostly seen in plants, animal and other living organisms. In human instance during fertilization haploid chromosome from one the parent cell combines with the other haploid chromosome from the other parent thus resulting into a diploid progeny possessing the character of both the father and mother. 2. Horizontal gene transfer: this is the transfer of genetic materials between bacteria cells uncoupled with cell division. When a gene is transferred between individuals of unrelated generations is said to be horizontal transfer. This type of transfer can occur by conjugation, transformation or transduction. TRANSFORMATION: Transformation involves the uptake of free or naked DNA released by donor by a recipient. It was the first example of genetic exchange in bacteria to have been discovered. This was first demonstrated in an experiment conducted by Griffith in 1928. Transformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain bacteria (e.g. Bacillus, Haemophilus, Neisseria, Pneumococcus) can take up DNA from the environment and the DNA that is taken up can be incorporated into the recipient's chromosome. The steps involved in transformation are: 1. A donor bacterium dies and is degraded 2. A fragment of DNA (usually about 20 genes long) from the dead donor bacterium binds to DNA binding proteins on the cell wall of a competent, living recipient bacterium. 3. Nuclease enzymes then cut the bound DNA into fragments. 4. One strand is destroyed and the other penetrates the recipient bacterium. 5. The Rec A protein promotes genetic exchange (recombination) between a fragment of the donor's DNA and the recipient's DNA. CONJUGATION: In 1946 Joshua Lederberg and Tatum discovered that some bacteria can transfer genetic information to other bacteria through a process known as conjugation. Bacterial conjugation is the transfer of DNA from a living donor bacterium to a recipient bacterium. Conjugation involves the transfer of plasmids from donor bacterium to recipient bacterium. Plasmids are small autonomously replicating circular pieces of double-stranded circular DNA. Plasmid transfer in Gram-negative bacteria occurs only between strains of the same species or closely related species. Some plasmids are designated as F factor (F plasmid, fertility factor or sex factor) because they carry genes that mediate their own transfer. The F factor can replicate autonomously in the cell. These genes code for the production of the sex pilus and enzymes necessary for conjugation. Cells possessing F plasmids are F+ (male) and act as donors. Those cells lacking this plasmid are F- (female) and act as recipient. All those plasmids, which confer on their host cells to act as donors in conjugation are called transfer factor. Each Gram negative F+ bacterium has 1 to 3 sex pili that bind to a specific outer membrane protein on recipient bacteria to initiate mating. The sex pilus then retracts, bringing the two bacteria in contact and the two cells become bound together at a point of direct envelope to-envelope contact. In Gram-positive bacteria sticky surface molecules are produced which bring the two bacteria into contact. Gram-positive donor bacteria produce adhesins that cause them to aggregate with recipient cells, but sex pili are not involved. DNA is then transferred from the donor to the recipient. Plasmid-mediated conjugation occurs in Bacillus subtilis, Streptococcus lactis, and Enterococcus faecalis but is not found as commonly in the Gram positive bacteria as compared to the Gram-negative bacteria. F+ conjugation: This results in the transfer of an F+ plasmid (coding only for a sex pilus) but not chromosomal DNA from a male donor bacterium to a female recipient bacterium. The two strands of the plasmid separate. One strand enters the recipient bacterium progressing in the 5' to 3' direction while one strand remains in the donor. The complementary strands are synthesized in both donor and recipient cells. The recipient then becomes an F+ male and can make a sex pilus. During conjugation, no cytoplasm or cell material except DNA passes from donor to recipient. The mating pairs can be separated by shear forces and conjugation can be interrupted. Consequently, the mating pairs remain associated for only a short time. After conjugation, the cells break apart. Following successful conjugation the recipient becomes F+ and the donor remains F+. TRANSDUCTION: Bacteriophages are viruses that parasitize bacteria and use their machinery for their own replication. During the process of replication inside the host bacteria the bacterial chromosome or plasmid is erroneously packaged into the bacteriophage capsid. Thus newer progeny of phages may contain fragments of host chromosome along with their own DNA or entirely host chromosome. When such phage infects another bacterium, the bacterial chromosome in the phage also gets transferred to the new bacterium. This fragment may undergo recombination with the host chromosome and confer new property to the bacterium. Life cycle of bacteriophage may either be lytic or lysogenic. In the former, the parasitized bacterial cell is killed with the release of mature phages while in the latter the phage DNA gets incorporated into the bacterial chromosome as prophage. The following are the stages of transduction involving a lytic phage: 1. A lytic bacteriophage adsorbs to a susceptible bacterium. 2. The bacteriophage genome enters the bacterium and the phage DNA directs the bacterium's metabolic machinery to manufacture bacteriophage components and enzymes. 3. Occasionally during maturation, a bacteriophage capsid incorporates a fragment of donor bacterium's chromosome or a plasmid instead of a phage genome by mistake. 4. The bacteriophages are released with the lysis of the bacterium. 5. The bacteriophage carrying the donor bacterium's DNA adsorbs to another recipient bacterium. 6. The bacteriophage inserts the donor bacterium's DNA it is carrying into the recipient bacterium. 7. The donor bacterium's DNA is exchanged by recombination for some of the recipient's DNA. The following are the stages of transduction involving a lysogenic phage: 1. A lysogenic bacteriophage adsorbs to a susceptible bacterium. 2. The bacteriophage genome enters the bacterium and the phage DNA is incorporated into a specific region in the bacteria chromosome (Prophage) where it replicates anytime the bacterial cell replicates.  MENDELIAN CONCEPT OF HEREDITARY Gregor Mendel who is today referred to as the father of genetics explained in his hybridization experiment that traits are inherited as particles which he was able to express numerically and subject to statistical analysis thus making it possible for scientists to predict the expression of trait using mathematical probability. This particulate material referred to by Mendel is now known as Gene; which is the unit of inheritance. In his experiment, pea plants were used because of its rapid growth rate being an annual crop, self-pollinating hence the ease of crossing and show varieties of contrasting traits (purple vs white flowers, tall vs short stems, round vs wrinkled seeds). Mendel cross pollinated true breeds of pea plant with pea plants of opposite trait (purple x white) which he called Parental generation (P). it was observed that all the offspring produce purple flower which he called First filial generation (F1) and F1 always display one trait, such expressed trait is called the dominant trait and it must have within it the trait from the original parents. Offspring from the F1 were self-pollinated and 3 of the offspring were found to have purple flower and 1 to have white flower which were termed second filial generation (F2). After Mendel self-fertilized the F1 generation and obtained the 3:1 ratio, he correctly theorized that genes can be paired in three different ways for each trait; AA, aa, and Aa. The capital A represents the dominant factor and lowercase a represents the recessive. Second filial generation is capable of expressing hidden trait and such trait is called recessive. Mendel therefore found out that each individual has two factors that determine the external appearance the offspring will exhibit such factor is now known as Alleles. The two factors may or may not contain the same information. If the two factors are identical, the individual is called homozygous for the trait. If the two factors have different information, the individual is called heterozygous. Crosses considering the inheritance of one feature only are called monohybrid crosses. Mendel also tried crosses involving two contrasting features, such as tall and red flowered with dwarf and white flowered plant such a cross is termed dihybrid cross. Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters. Without his careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics. Mathematically, Mendel was able to prove his experiment AA aa P-generation X A A a a Alleles X F1 Aa Aa Aa Aa At the end F1, all has purple flower pea plant Aa Aa F1 X A a A a Alleles X F2 Aa AA aa Aa Phenotypic expressed trait from the self pollinated F1 offspring show 3 pea plant with purple flower and 1 pea plant with white flower hence 3:1 This experiment can also be expressed using punnett square as shown below X P P p Pp Pp p Pp Pp Phenotypically at the end of F1 all the plants were purple X P P P PP Pp p Pp pp Phenotypically at the end of F2 3 plants were purple and 1 white (3:1) This first kind of experiment conducted by Mendel was a monohybrid experiment i.e crossing with one pair of contrasting trait. In dihybrid experiment (crossing with two pairs of contrasting traits), Mendel crossed pea plants that have wrinkled seeds (rr) and white colour (yy) with pea plants that have smooth seeds (RR) and purple colour (YY) and the following were observed First filial generation give rise to offsprings that have purple colour with smooth seeds Self pollination of offspring from the F1 generation gives rise to 9 purple flower and smooth seed; 3 white flower and smooth seed; 3 purple flower and wrinkled seed and 1 white and wrinkled seed (9:3:3:1). MENDEL’S LAWS The set of three laws, proposed by Gregor J. Mendel in the mid-1860s, to explain the biological inheritance or hereditary is known as Mendel’s laws. These laws are the law of segregation, law of independent assortment, and law of dominance, and they form the core of classical genetics till date. His scientific results describe the transmission of hereditary information from parent generation to progeny generation. These results helped him to frame three laws of biological inheritance, which led to the foundation of classical genetics First Law: Law of Dominance When individual with one or more set of contrasting characters are crossed then the character that appear in F1 generation are called Dominance characters and the characters that remain hidden are called recessive characters. From the illustration below where P generation plant were crossed together and all the offsrpings were observed to be purple , shows that the dominant purple flower allele (B) will hide the phenotypic effects of the recessive white flower allele (b). This is known as the law of dominance. White phenotypes will appear only in the absence of dominant purple flower alleles. The uppercase letters are used to denote dominant alleles, whereas the lowercase letters are used to denote recessive alleles. Mendel used the term “factors” instead of alleles during that time. The dominance and recessive of genes can be explained on the basis of enzymatic functions of genes. The dominant genes - are capable of synthesizing active polypeptides or proteins that form functional enzymes, whereas the recessive genes (mutant genes) code for incomplete or non-functional polypeptides. Therefore, the dominant genes produce a specific phenotype while the recessive genes fail to do so. In the heterozygous condition also the dominant gene is able to express itself, so that the heterozygous and homozygous individuals have similar phenotype. Second Law: Law of Segregation (Purity of gametes) The law of segregation states that when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid, the two separate out from each other and only one enters each gamete. During reproduction, the inherited factors (now called alleles) that determine traits are separated into reproductive cells by a process called meiosis and randomly reunite during fertilization. This law is also referred to as law of purity of gametes. During the formation of male and female gametes (generally sperm and ova in animals or pollen grains and ovule in plants), factors (alleles) responsible for a particular character separate and are passed into different gametes. This process implies that the gametes are either pure for dominant alleles or for recessive. These gametes can unite randomly in different possible combinations during fertilization and produce the genotype for the traits of the progenies. In a zygote, the two members of an allele pair remain together without being contaminated. This is known as law of segregation. Example - Pure tall plants are homozygous and, therefore/possess genes (factors) TT; similarly dwarf possess genes tt. The tallness and dwarfness are two independent but contrasting factors or determiners. Pure tall plants produce gametes all of which possess gene T and dwarf plants t type of gametes. During cross fertilization gametes with T and t unite to produce hybrids of F 1 generation. These hybrids possess genotype Tt. It means F1 plants, though tall phenotypically, possess one gene for tallness and one gene for dwarfness. Apparently, the tall and dwarf characters appear to have become contaminated developing only tall character. But at the time of gamete formation, the genes T (for tallness) and t (for dwarfness) separate and are passed on to separate gametes. As a result, two types of gametes are produced from the heterozygote in equal numerosity. 50% of the gametes possess gene T and other 50% possess gene t. Therefore, these gametes are either pure for tallness or for dwarfness. (This is why the law of segregation is also described as Law of purity of gametes). Gametes unite at random and when gametes are numerous all possible combinations can occur, with the result that tall and dwarf appear in the ratio of 3 :1. The results are often represented by Punnett square as follows: RR have only gene for round Rr, rR have gene for round and wrinkle rr have only wrinkled gene Third Law: Law of Independent Assortment This law states that alleles of different genes assort independently of one another during gamete formation i.e genes located on different chromosomes will be inherited independently of each other. This law is also known as inheritance law. Mixing a single trait (monohybrid cross) in Mendel’s experiment constantly resulted in a 3:1 ratio between dominant and recessive phenotypes. However, when he performed experiments on two traits (dihybrid cross), he obtained F2 generation in the ratio of 9:3:3:1. These results led Mendel to conclude that different traits (e.g., seed shape and colour) are inherited independently of one another and there is no relation between two traits. Independent assortment occurs during meiosis I in eukaryotic organisms, specifically metaphase I of meiosis, to produce a gamete with a mixture of the organism's maternal and paternal chromosomes. Along with chromosomal crossover, this process aids in increasing genetic diversity by producing novel genetic combinations. In independent assortment the chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 223 or 8,388,608 possible combinations. The gametes will normally end up with 23 chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny. From below diagram yellow and round characters are dominant over green and wrinkled characters which can be represented as follows: (i) gene for yellow colour of cotyledons Y (ii) gene for green colour of cotyledons y (iii) gene for round character of cotyledons R (iv) gene for wrinkled character of colyledons r  Therefore, plants with yellow and round cotyledons will have their genotype YYRR and those with green and wrinkled cotyledons will have a genotype yyrr. These plants will produce gametes with gene YR and yr respectively. When these plants are cross pollinated, the union of these gametes will produce F1 hybrids with YyRr genes. When these produce gametes all the four genes have full freedom to assort independently and, therefore, there are possibilities of four combinations in both male and female gametes. (i)RY (ii) Ry (iii) rY (iv) ry This shows an excellent example of independent assortment. These gametes can unite at random producing in all 16 different combinations of genes, but presenting four phenotypes in the ratio of 9:3:3:1. BIOLOGICAL SIGNIFICANCE OF MENDEL'S LAWS Mendel's work remained buried for about three decades, but after its rediscovery, the laws are being used for the various branches of breeding. These are used for improving the varieties of fowls and their eggs; in obtaining rust-resistant and disease-resistant varieties of grains. Various new breeds of horses and dogs are obtained by cross breeding experiments. The science of Eugenics is the outcome of Mendelism, which deals with the betterment of human race. Mendelian Deviation Mendelian deviations or exceptions or anomalies includes 1) Incomplete dominance 2) Codominance 3) Lethal genes etc. 1. Incomplete dominance Mendel always observed complete dominance of one allele over the other for all the seven characters, which he studied, in garden pea. Later on cases of incomplete dominance were reported. For example, in four ëoí clock plant (Mirabilis jalapa) there are two types of flower viz., red and white. A cross between red and white flowered plants produced plants with intermediate flower colour i.e. pink colour in F1 and a modified ratio of 1 red: 2 pink: 1 White in F2. Parents Red flower x White flower RR x rr F1 Rr pink flower F2 1 Red (Rr) : 2 Pink (RR) : 1 White (rr) 2. Codominance In case of codominance both alleles express their phenotypes in heterozygote greater than an intermediate one. The example is AB blood group in human. The people who have blood type AB are heterozygous exhibiting phenotypes for both the IA and IB alleles. In other words, heterozygotes for codominant alleles are phenotypically similar to both parental types. The main difference between codominance and incomplete dominance lies in the way in which genes act. In case of codominance both alleles are active while in case of incomplete dominance both alleles blend to make an intermediate one. Codominance- both genes fully expressed 3. Lethal genes Gene, which causes the death of its carrier when in homozygous condition is called lethal gene. Mendel’s findings were based on equal survival of all genotypes. In normal segregation ratio of 3:1 is modified into 2:1 ratio. Lethal genes have been reported in both animals as well as plants. In mice allele for yellow coat colour is dominant over grey. When a cross is made between yellow and grey a ratio of 1:1 for yellow and gray mice was observed. This indicated that yellow mice are always heterozygous. Because yellow homozygotes are never born because of homozygous lethality. Such genes were not observed by Mendel. He always got 3:1 ratio in F2 for single gene characters. Lethal genes can be recessive, as in the aforementioned mouse experiments. Lethal genes can also be dominant, conditional, semilethal, or synthetic, depending on the gene or genes involved. Backcrossing Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. Test crossing Since some alleles are dominant over others, the phenotype of an organism does not always reflect its genotype. A recessive phenotype (yellow) is only expressed when the organism is homozygous recessive (gg). A pea plant with green pods may be either homozygous dominant (GG) or heterozygous (Gg). To determine whether an organism with a dominant phenotype (e.g. green pod color) is homozygous dominant or heterozygous, test cross is carried out. The breeding of an organism of unknown genotype with a homozygous recessive. If all the progenies of the test cross have green pods, then the green pod parent was probably homozygous dominant since a GG x gg cross produces Gg progeny. If the progeny of the testcross contains both green and yellow phenotypes, then the green pod parent was heterozygous since a Gg x gg cross produces Gg and gg progeny in a 1:1 ratio. The testcross was devised by Mendel and is still an important tool in genetic studies.